Katie Ledecky of the US listens to the Star Spangled Banner after winning the 1500m freestyle at the world championships. Photograph: Fabrice Coffrini/AFP/Getty Images

The American teenager Katie Ledecky smashed the world record for the 1500m freestyle to take her second gold medal, after the 800m, at the world championships on Tuesday in Barcelona. Looking stronger at the end of the gruelling race than she did at the beginning, the 16-year-old obliterated the previous record by more than six seconds.

Missy Franklin, also of the US, picked up a second gold medal as well. Cruising through a demanding double, the 18-year-old who made her name at the London Olympics easily won the 100m backstroke, then returned about an hour later to post the second-fastest time in the semi-finals of the 200m free. "It's tough but it's fun," Franklin said. "I'm super happy with my 100 back. It really got me pumped up for the 200 free."

It was a good night for the Americans after they failed to win gold the previous day.

Matt Grevers and David Plummer went 1-2 in the 100 backstroke for the third U.S. victory of the session. Conor Dwyer picked up a silver behind France's Yannick Agnel in the 200 free, and Jessica Hardy chipped in with a bronze in the 100 breaststroke won by Lithuania's Ruta Meilutyte.

The only disappointment for the US was Ryan Lochte, who laboured to fourth in the men's 200m freestyle. "It wasn't my night," the five-times Olympic gold medallist said. "But I have to put it behind me because I still have many races to swim."